In various printing technologies, marking material is applied to the surface of an intermediate imaging element, such as a belt or a drum. The print media to which the image is ultimately to be applied (such as paper) is then pressed against the intermediate imaging element to transfer the image from the intermediate imaging element to the print media. In one example using electrostatographic or xerographic printing, an image of ink liquid or dry toner) is formed on an electrically charged image receptor. The print media is pressed against the image receptor to transfer the image to the print media. The image is subsequently fused to the print media by applying pressure with a fuser roller. In another example using phase change ink jet printing, ink is deposited to form an image on the surface of an imaging drum. A transfix roller presses the print media against the image-bearing drum surface to transfer the ink image from the drum surface to the print media and fuse the ink image to the print media.
In many circumstances, it is desirable for the pressure applied to be constant, regardless of the thickness of the print medium. Therefore, displacement of the pressure applicator due to different thicknesses of print medium should not materially change the magnitude of the pressure applied. Furthermore, it is often desirable that the pressure applied be balanced across the width of the print medium.
In accordance with one aspect of the present invention, an image transfer mechanism for pressing a print medium against an imaging element includes a pressure element and a lever system for pressing the pressure element toward the imaging element. The lever system has a load attachment point that has a range of position that depends on the thickness of a print medium positioned between the imaging element and the pressure element. A load mechanism includes a load connector with a proximal end and a distal end, with the distal end attached to the load attachment point of the lever system so that displacement of the lever system attachment point causes longitudinal movement of the load connector. The load mechanism applies at the lever system load attachment point a load that is substantially constant throughout the range of position of the lever system load attachment point. The load mechanism includes a spring and a crank attached to the spring and to the proximal end of the load connector so that longitudinal movement of the load connector causes a change in the length of the spring. The crank geometry is configured so that a change in the spring force due to longitudinal movement of the load connector produces a lesser change in a load force at the distal end of the load connector than the change in the force of the spring due to the change in spring length.
Another aspect of the present invention includes a load mechanism for applying a load force, with the load mechanism including a crank having a crank pivot, a spring attached to the crank at a spring attachment, and a load connector attached to the crank at a load connector attachment. The spring attachment and the load connector attachment are separated by an attachment angle relative to the crank pivot, and the spring has a spring direction of action relative to the crank. The spring direction of action has a spring effective radius extending perpendicular to the spring direction of action from the crank pivot to the spring direction of action, while the load connector has a load direction of action relative to the crank. The load connector direction of action has a load connector effective radius extending perpendicular to the load connector direction of action from the crank pivot to the load connector direction of action, and the spring effective radius and the load connector effective radius are separated by an action separation angle. The action separation angle is different from the attachment angle.
In yet another aspect, the present invention includes a load mechanism for applying a load force, with the load mechanism including a crank having a crank pivot, a spring attached to the crank at a spring attachment, and a load connector attached to the crank at a load connector attachment. The spring attachment and the load connector attachment are separated by an attachment angle relative to the crank pivot, and the spring has a spring direction of action relative to the crank. The spring direction of action has a spring effective radius extending perpendicular to the spring direction of action from the crank pivot to the spring direction of action, while the load connector has a load direction of action relative to the crank. The load connector direction of action has a load connector effective radius extending perpendicular to the load connector direction of action from the crank pivot to the load connector direction of action, and the spring effective radius and the load connector effective radius are separated by an action separation angle. As the crank rotates in a first rotational direction, the length of the load connector effective radius and the length of the spring effective radius change at different rates.